In the belted radial ply tires, especially which is provided with a stiff steel belt, the ground pressure distribution is liable to become uneven. As a result, uneven wear is liable to occur between the tread crown and tread shoulders. In general, the amount of wear in the tread shoulder is larger than that of the tread crown. Such uneven wear is called shoulder wear.
The shoulder wear is caused by a difference in the tire diameter between the tread crown and tread shoulders due to a curved tread profile. This causes a slip against the ground and the tread shoulders are worn earlier than the tread crown.
Further, a so called heel and toe wear is liable to occur in the tread shoulder, which wear is an uneven wear in the front and rear edges of tread elements such as blocks.
Such uneven wears can be reduced by making the ground pressure distribution even across the whole of the ground contacting width and thereby making the slip amount distribution even.
It has been therefore, considered that the uneven wear can be reduced by increasing the radius of curvature of the tread profile.
However, it was found that when the radius of curvature is excessively increased, the ground pressure is increased in the tread shoulder. As a result, the shoulder wear increases during cornering, and the heel and toe wear increases in the tread crown portion during straight running.
On the other hand, as a counter measure to such uneven ground pressure distribution, many propositions to provide a specific radius of curvature for the belt have been made, for example, in U.S. Pat. No. 4442879, U.S. Pat. No. 4702293, U.S. Pat. No. 4842682, U.S. Pat No. 5061667, CA-A 2018217, EP-A 323519, EP-A 490596, and Japanese Patent Application No. 58-170602.
It was, however, found that even when the belt radius is specifically defined as proposed, the ground pressure distribution is not necessarily even. For example, it was often observed that even when the belt edge portion is decreased in the radius of curvature in comparison with the center portion, the ground pressure is rather increased in the shoulder portion, and uneven wear is not improved.
The present inventors discovered that the ground pressure distribution varies depending on not only the radius of curvature of the belt but also the tread rubber thickness, tread profile and carcass profile in the shoulder portion, and then the present invention was made.